🔍 Key Concepts

• L1 and L2 GPS carrier frequencies and what each one is primarily used for • What is meant by the navigation message and the SPS (Standard Positioning Service) code • How dual-frequency reception helps estimate ionospheric delay

💭 Think About

• Which option correctly states the standard L1 and L2 frequencies used by GPS satellites? Check the exact MHz values first. • Ask yourself: Does the same carrier usually carry BOTH the navigation message and the SPS code, or are they split between different carriers? • Consider how ionospheric delay is actually measured: is it by transmitting a special "delay" signal, or by comparing how the same code appears on two different frequencies?

✅ Before You Answer

• Verify the exact L1 frequency (around 1575 MHz) and the exact L2 frequency (around 1227 MHz). Eliminate any option with clearly wrong numbers. • Check which carrier is associated with the Standard Positioning Service (SPS) code and the civil navigation message. • Make sure the option you pick does NOT claim that the two carriers are physically "combined" into one transmitted signal in the satellite hardware—understand that they are transmitted as separate carriers.